Butyl rubber is known for its excellent insulating and gas barrier properties. Generally, commercial butyl polymer is prepared in a low temperature cationic polymerization process using Lewis acid-type catalysts, such as aluminum trichloride. The process commonly used employs methyl chloride as the diluent for the reaction mixture and the polymerization is conducted at temperatures less than −90° C., resulting in production of a polymer in a slurry of the diluent.
Alternatively, it is possible to produce the polymer in a diluent, which acts as a solvent for the polymer (e.g., hydrocarbons such as pentane, hexane, heptane and the like). The product polymer may be recovered using conventional techniques in the rubber manufacturing industry.
In many of its applications, butyl rubber is used in the form of cured compounds. Vulcanizing systems usually utilized for butyl rubber include sulfur, quinoids, resins, sulfur donors and low-sulfur high performance vulcanization accelerators. However, sulfur residues in the compound are often undesirable, e.g., they promote corrosion of parts in contact with the compound.
High performance applications of butyl rubber, like condenser caps or medical devices, require halogen- and sulfur-free compounds. The preferred vulcanization system in this case is based on peroxides since this produces an article free of detrimental residues. In addition, peroxide-cured compounds offer higher thermal resistance and other advantages compared to sulfur-cured materials.
If peroxides are used for cross-linking and curing of conventional butyl rubbers, the main chains of the rubber degrade and satisfactorily cured products are not obtained.
One way of obtaining peroxide curable butyl rubber is to use a regular butyl rubber with a vinyl aromatic compound like divinylbenzene (DVB) and an organic peroxide, as described in JP-A-107738/1994. Another similar way to obtain a partially cross-linked butyl rubber is to use a regular butyl rubber with an electron withdrawing group-containing polyfunctional monomer (ethylene dimethacrylate, trimethylolpropane triacrylate, N,N′-m-phenylene dimaleimide, etc.) and an organic peroxide, as disclosed in JP-A-172547/1994. The disadvantage of these methods is that the resulting compound is contaminated with the low molecular weight reagents added to induce cross-linking, which did not fully react with the rubber in the solid state. Also, the action of peroxide on the regular butyl rubber may lead to formation of some low molecular weight compounds from the degraded rubber. The final articles based on such compounds may display an undesirable characteristic of leaching out the low molecular species and accelerated aging
A preferred approach nowadays is to use a commercial pre-crosslinked butyl rubber such as commercially available Bayer® XL-10000 (or, formerly XL-20 and XL-50) that can be cross-linked with peroxides, e.g., see Walker et al., “Journal of the Institute of the Rubber Industry”, 8 (2), 1974, 64–68. XL-10000 is partially cross-linked with divinylbenzene already in the polymerization stage. No peroxides are present during this polymerization process which takes place via a cationic mechanism. This leads to a much ‘cleaner’ product than the partially cross-linked butyl disclosed in JP-A-107738/1994. In the latter case, the curing has to be continued for sufficiently long time so that both functional groups of the DVB molecules react and are incorporated into polymer chains.
While said commercial pre-cross-linked polymers exhibit excellent properties in many applications, they have a gel content of at least 50 wt. % which sometimes makes the even dispersion of fillers and curatives normally used during vulcanization difficult. This increases the likelihood of under- and over-cured areas within the rubbery article, rendering its physical properties inferior and unpredictable. Also, the Mooney viscosity of this rubber is high, usually 60–70 units (1′+8′@125° C.) which may cause significant processing difficulties, especially in mixing and sheeting stages.
British Patent 1,354,340 described a solution method for producing homo- or copolymers of isoolefins (comprising a copolymer of isobutylene and DVB)-using a mixed catalyst system composed of metal oxide/aluminum alcoholate and boron trifluoride. The process could be carried out in methyl chloride or in a hydrocarbon solvent, usually at −65° C. or −75° C. The content of DVB in the monomer feed could vary between 0.5% to 30%, by weight. The products had high molecular weights. However, this process is believed to result in a polymer with a high gel content due to a high amount of DVB in the feed and its uncontrolled reactions.
RU 2,130,948 discloses the copolymerization of isobutylene with DVB in an aromatic or aliphatic hydrocarbon solvent initiated with a system comprising TiCl4 and triisobutylaluminum. The content of DVB in the monomer feed was 0.1 to 5.0 wt. %, based on isobutylene. The process was to be carried out in the temperature range −40 to +40° C. The products had low molecular weights (Mv<15,000 g/mol) and were useful as additives for thickening of lubricants. The process of the present invention operates at a lower polymerization temperature and the viscosity average molecular weight of the product is higher, about 125,000–210,000 g/mol.
U.S. Pat. No. 5,395,885 discloses a multi-arm radial-star polyisobutylene obtained by the addition of excess DVB cross-linking reagent to a living polyisobutylene charge, i.e., by the “arm first” method under specific conditions. The polymerization was induced by the TiCl4/2-chloro-2,4,4-trimethylpentane initiating system in the presence of triethylamine as the electron pair donor. The reactions were performed in methylene chloride/hexanes mixtures (50:50 vol.). The star polymer could be useful as a viscofier, particularly for oils, which could be obtained without the need for a post-polymerization hydrogenation step. The synthesis and the structure of this polymer are significantly different from those described in the present invention (two-stage vs. one-stage process and a star-shaped vs. random short branched/slightly crosslinked elastomer, respectively). Also, the synthesis of the multi-arm polymer was based on a living isobutylene polymerization, which is not the case in the present invention. In fact, in these special star-shaped polymers the presence of ‘traditional’ isobutylene-divinylbenzene polymers was undesirable and would be treated as an impurity. The content of an aromatic core in a representative star-PIB polymer was about 22 wt. %. This core was composed of homopolymerized divinylbenzene, including crosslinked species. The fact that the star polymers had virtually no residual unsaturation subsequent to star formation indicates that most likely the degree of crosslinking was high. The lack of pendant double bonds from DVB in the polymer would make it unsuitable for applications involving peroxide cure.
Furthermore, the above examples were not involved with peroxide cured compounds of the isobutylene-divinylbenzene copolymers.
Canadian Patent 817,939 teaches that in order to have peroxide-curable butyl-type polymer, the presence of an aliphatic diene, like isoprene, is not necessary in the polymerization mixture. However, the presence of an aliphatic diene can have a moderating influence on the course of polymerization thus providing a means whereby the molecular weight of the polymer can be controlled. Especially suitable amounts of isoprene are from 1% to 5% by weight of the monomer mixture comprising isobutylene, isoprene and divinylbenzene monomers. The preferred solvent is methyl or ethyl chloride, a Friedel-Crafts catalyst, such as aluminum chloride, and temperature preferably in the range −40° C. to −110° C. The especially useful content of an aromatic divinyl compound in the monomer feed is 0.5% to 3% by weight (in the neat form). The resulting polymers had a much-reduced tendency to cold flow over “regular” butyl rubbers made from the monoolefin and an aliphatic conjugated diene. This was the result of crosslinks introduced by the presence of divinyl aromatic compound in the unvulcanized polymer. Because of the existence of such crosslinks, the polymers were referred to as “cross-linked butyl” throughout the specification, which would not fulfill the requirement that polymer contains less than 15 wt. % of solid matter insoluble in boiling cyclohexane under reflux for 60 min. The above applications did not involve the presence of a chain transfer agent in the monomer feed during polymerizations.
Processability-improving polymers are often added to the pre-crosslinked butyl rubber to overcome some of these problems. Such polymers are particularly useful for improving the mixing or kneading property of a rubber composition. They include natural rubbers, synthetic rubbers (for example, IR, BR, SBR, CR, NBR, IIR, EPM, EPDM, acrylic rubber, EVA, urethane rubber, silicone rubber, and fluororubber) and thermoplastic elastomers (for example, of styrene, olefin, vinyl chloride, ester, amide, and urethane series). These processability-improving polymers may be used in the amount of up to 100 parts by weight, preferably up to 50 parts by weight, and most preferably up to 30 parts by weight, per 100 parts by weight of a partially cross-linked butyl rubber. However, the presence of other rubbers dilutes said desirable properties of butyl rubber.